As will be used throughout this application, the x, y and z axes will be referred to as the x-elevation axis, the y-azimuthal axis, and the z-range axis. FIG. 1 is a perspective view of a transducer assembly according to the prior art. The transducer assembly 10 includes a backing block 12, a layer of transducer material 14, a first matching layer 16, a second matching layer 18, a window 20 and a plastic housing 22. Also included is a first flex circuit 24 disposed on one surface of the transducer layer 14 and a second flex circuit 26 disposed on an opposite surface of the transducer layer 14 as is well known. The layer of transducer material 14 has a thickness measured in the z-range direction that is constant at each point along the x-elevation axis. The layer of transducer material 14 is flat in the x-elevation direction, i.e., has no curvature. Because the layer of transducer material 14 is flat and has a constant thickness, in order to provide geometrical focusing of an emitted ultrasound beam in the elevation plane, the window 20 disposed over the transducer must be a focusing lens, preferably of a room temperature vulcanized (RTV) silicon material. There are disadvantages associated with such a transducer. The window 20 material is exposed to the environment and thus is exposed to various chemicals including coupling gels when the transducer is in use and disinfectants for cleaning the transducer after use which degrade the integrity of the window. In addition, because the window of the transducer is physically placed against and moved along a surface of the body during imaging, the window is subject to mechanical forces which also degrade the integrity of the window. Also, because the window 20 is formed separately from the plastic housing 22 and is bonded thereto, the bond may be defective or deteriorate over time thus allowing leakage into the housing 22 itself. These chemical and mechanical forces impose stresses on the window of the transducer which introduce problems in the reliability of the transducer and a deterioration in its performance over time.
FIG. 2 is a cross-sectional view taken along the x-elevation axis of another transducer assembly according to the prior art. The transducer assembly 110, like the transducer assembly 10 shown in FIG. 1, includes a backing block 112, a layer of transducer material 114 of uniform thickness, a first acoustic matching layer 116, a second acoustic matching layer 118, a window 120, a plastic housing 122 and a first and second flex circuit 124, 126. Unlike the transducer assembly 10 shown in FIG. 1, the layer of transducer material 114 is curved in the x-elevation direction so that both the front and back surfaces 113, 115 are concave in shape. The curvature of the layer of transducer material 114 provides focusing of an emitted ultrasound beam along the x-elevation axis. Such transducers are known as mechanically focused transducers. Other mechanically focused transducers use a layer of transducer material that has a non-uniform thickness. See U.S. Pat. Nos. 5,438,998 and 5,415,175. Because the layer of transducer material 114 itself provides the focusing, the window material 120 need not be made of focusing material. Like the transducer assembly shown in FIG. 1, however, the window 120 is exposed to the environment and separately bonded to the housing 122 and thus suffers from the same disadvantages.
U.S. Pat. No. 5,562,096 ("the '096 patent") discloses an ultrasonic transducer probe with an axisymmetric lens. The reference numerals in this paragraph refer to the reference numerals in the '096 patent. The transducer probe includes a uniform, flat layer of transducer material 4 with a lens 6 disposed thereover both of which are located in an integrated housing 2. In the region of the housing 2 that is adjacent to the lens 6, the continuous surface 10 of the transducer housing 2 is formed into a window 12. In one embodiment, when a lens 6 is disposed in the housing 2, the thickness of the window 12 is constant. Alternatively, the thickness of the window 12 may vary as a function of distance from axis 8. The window may be formed into an axisymmetric converging lens by increasing the thickness of the window as a function of distance from axis 8. Similarly, the window may be formed into an axisymmetric diverging lens by decreasing the thickness of the window as a function of increasing distance from axis 8. In either of these embodiments the probe can be constructed with or without the lens 6. Preferably, the transducer housing 2 is formed from an acoustically transmissive material such as a thermoplastic material and more particularly, TPX.TM. from Mitsui Petrochemicals (America).
U.S. Pat. No. 4,387,720 discloses a transducer that includes a flat, uniform thickness array of crystals 2, a focusing lens 15 disposed over the crystals and a non-focusing filler disposed between the transducer layer and the lens. The lens element 14 may be formed of polymethylpentene, polyethylene and polypropylene, all modified with a rubber modifier such as ethylene propylene. The lens element 14 has an outer face 15 that may be flat or slightly curved. An inner lens element 24 is disposed between the outer lens element 14 and a shield 8 and may be formed of a potting compound such as urethane. The outer lens element 14 is a separate piece from nose piece 26. The curvature of the inner surface 15' of the outer lens element 14 provides the focusing of an emitted ultrasound beam in the elevation plane.
It is thus desirable to provide an ultrasound transducer that has a reliable chemical resistant and high impact resistant shell. It is also desirable to provide an ultrasound transducer that has a seamless design to reduce the possibility of leakage of chemicals into the transducer housing. It is also desirable to provide a transducer that is blood compatible.